Vehicle drivetrains for providing four-wheel driving have been developed in the past to provide increased traction and the consequent ability to negotiate roads and other terrain on which travel would not be possible with the more conventional two-wheel driving. Certain four-wheel drivetrains drive all four wheels without any differential action between the front and rear axles. As such, these vehicles have good mobility under adverse terrains but are not particularly driveable on normal road conditions. This is because such drivetrains drive the front and rear wheels at the same speed such that turning results in sliding and/or scuffing of the wheels. To overcome such problems, transfer cases are utilized to selectively disconnect one pair of wheels while maintaining the driving of the other pair of wheels.
U.S. Pat. Nos. 3,923,113 and 4,031,780 disclose vehicle drivetrains which were developed to permit four-wheel driving with a differential operating between the front and rear wheels in order to permit road driving with all four wheels even when there is a difference in speed between the front and rear wheels such as is involved with turning. To prevent excessive speed differences between the front and rear wheels and possible slippage, the differentials utilized with such drivetrains are normally of the limited slip type similar to those used with vehicle axles to prevent slippage of the left or right wheel as the other remains stationary.
United Kingdom Pat. No. 1,357,106 discloses a vehicle drivetrain that normally drives through the rear wheels but has a viscous control coupling for driving the front wheels when there is a significant speed difference between the front and rear wheels. This viscous control coupling prevents rear wheel slippage by driving through the front wheels and also inhibits front or rear wheel locking.
Until the recent introduction of front-wheel drive vehicles, most vehicles included a front engine for driving a transmission and a prop shaft that connected the output of the transmission with a rear wheel differential for providing rear wheel driving. With such drivetrains, a transfer case is interposed between the transmission and the rear wheel differential and has an output that is selectively connected to the front wheels to provide front wheel driving. A differential can be conveniently incorporated into the transfer case to provide differential action between the front and rear wheels in order to accommodate for difference in travel such as is required in normal road driving.
Recently introduced front-wheel drive vehicles include a transaxle whose input is driven by the vehicle engine and whose output drives the adjacent front wheels. With such transaxle drivetrains, transfer cases are not conventionally incorporated to additionally permit rear wheel driving under road conditions since incorporation of a differential between the front and rear wheels is not readily possible. Rather, such a transaxle drivetrain for permitting four-wheel driving would require that the output from the transaxle for driving the front wheels first pass through a torque path through a differential in the transfer case and then back through the transaxle to the front wheels. Such a construction can be achieved by the use of a central output shaft and concentric sleeves that provide the torque path to the transfer case differential, but this construction is somewhat involved and adds substantial cost.